The invention relates to a switched mode power supply, especially for television receivers, the supply having an inductor and clamping diode in combination with a switching transistor whose duty cycle is varied at the horizontal scanning rate, to regulate an output voltage level of a power supply.
Switching regulators use the on and off time of a switching transistor to regulate the current supplied to a load circuit. By varying the switching time instead of varying the conducting current level, the switching regulator avoids the high power dissipation of series regulators. In television apparatus, a switching regulator can be used in conjunction with the flyback transformer that generates horizontal beam deflection signals. The switching regulator is operated synchronously with horizontal scanning, but the on-time or duty cycle of the switching regulator is varied as needed to couple sufficient power to secondary windings of the flyback transformer for regulating various supply voltages to maintain reference levels. This includes the regulated B+ supply voltage, which supplies current to a winding of the flyback transformer coupled to the horizontal output transistor for effecting horizontal beam scanning.
A switching regulator as described can be driven by a pulse width modulator in a feedback arrangement. A current supply charges a capacitor repetitively at the horizontal rate to provide a sawtooth voltage that is applied to one input of a comparator. An error voltage representative of the regulated output voltage is applied to another input of the comparator. The comparator controls the switching transistor of the voltage regulator, namely turning off the switching transistor at the time during each period when the sawtooth voltage exceeds the regulated output voltage. The duty cycle of the switching transistor is made relatively higher to couple more power through the switching transistor when needed, and vice-versa, to maintain the output voltage at a reference level.
In one type of switching regulator, known as a "buck" regulator, the switching transistor and an inductor are coupled in series between the unregulated supply voltage and an output, typically with a parallel storage capacitor. An electromagnetic field is built up in the inductor when the transistor is conducting. When the switching transistor turns off, back electromotive force induces a voltage across the inductor. A clamping diode or "catch" diode is coupled to hold the junction of the switching transistor and the inductor at voltage just below ground. The back electromotive force then is applied to maintain the positive voltage on the output, with the inductor supplying current to maintain the output voltage when the switching transistor is off. This arrangement relies on the switching transistor to turn off current to the inductor, to generate an inductive power impulse, also referred to as an inductive buck.
One of the difficulties encountered in the design of a buck type regulator is the need to drive the gate of the switching transistor. Where the switching transistor is a MOSFET, for example, the gate voltage must be referenced to the MOSFET source terminal. However, the voltage at the source terminal is not constant, instead varying with the input voltage level relative to ground depending on the conduction of the MOSFET.
In the conventional buck regulator, the inductor and catch diode are coupled to the source (or emitter) of the switching transistor. That is, the transistor is upstream of the inductor along the current supply path, between the unregulated supply voltage and the junction of the inductor and catch diode. When the transistor switches off, the source terminal of the transistor and the cathode of the catch diode attempt to go negative as the back electromotive force that arises on the inductor, is subtracted from the regulated output voltage at the opposite terminal of the inductor. Conduction of the catch diode then clamps the voltage at the upstream terminal of the inductor. The inductive impulse can then be applied to the output.